1. Field of the Invention
The invention is directed toward new methods for estimating and imaging the spatial and temporal mechanical behavior of materials in response to a mechanical stimulus. These methods are designed to work in inherently noisy applications, such as the imaging of the time-dependent mechanical behavior of biological tissues in vivo and using a preferred hand-held configuration of scanning.
Embodiments of the invention overcome the limitation of current elastographic methods for imaging local strains and displacements in inherently noisy environments, which are primarily due to echo decorrelation problems generated by uncontrollable motion. Embodiments of the invention minimize the decorrelation noise between the ultrasonic frames used for the generation of the elastograms since the reference pre-compression frame is continuously moved in time and the inter-frame time interval is maintained sufficiently short during the entire acquisition. This allows the generation of good quality elastograms for short (sub-second) as well as long (multi-second) acquisition times. In addition, from the time-dependent behavior of the local strains or displacements occurring in the material, images of local strain time constants and local displacement time constants can be generated using curve-fitting techniques.
2. Description of the Prior Art
Prior art techniques for making time-dependent elastographic measurements require the use of a fixed pre-compression RF frame that is acquired immediately before compression and post-compression frames that are acquired sequentially at increasing time-intervals with respect to the fixed pre-compression frame. Elastograms are then generated by applying elastographic techniques between the same pre-compression frame and the successive post-compression frames. This methodology has been proven to be not adequate for imaging the temporal behavior of materials in inherently noisy environments because of the echo decorrelation problems that are encountered due to uncontrolled motion, which may be significant shortly after compression. Embodiments of the present invention overcomes the limitations of the aforementioned techniques because the elastograms are generated using frames that are sufficiently close in time to avoid decorrelation due to uncontrollable motion.
Prior art elastographic methods used to generate axial elastograms in vivo are focused on the determination of tissue's axial displacements and strains after the application of a compression. These displacements or strains are computed by using a frame that is acquired immediately before the application of the compression and a frame that is acquired immediately after the application of the compression. To minimize noise, usually the compression is divided in a multiplicity of small compression steps and at the end of each step an echo sequence is acquired. Axial displacements or strain are generated using the various echo-sequences acquired during the compression. In general, the axial displacement or strains are then averaged to reduce noise.
These prior art methods may allow obtaining axial displacement and strain of adequate quality, in vivo, but they may not allow estimating the time-dependent mechanical changes occurring in such displacements and strains in materials that exhibit mechanical properties that vary with time. Indeed the usual assumption of these prior art methods is that the target body can be modeled as a purely linearly elastic material, so that no significant time-dependent mechanical changes occur during the acquisition of the echo-sequences.
The present invention differs from the aforementioned prior art techniques because the mechanical stimulus is first applied to the target body and thereafter the echo-sequences used for determining the displacements or strains are acquired. In the present invention the time-dependent mechanical behavior of a material after the application of a mechanical stimulus is imaged by means of post-stimulus echo-sequences only. As such, the method of this invention is directed toward materials that exhibit a time dependent mechanical behavior in response to the applied mechanical stimulus. In addition, the present invention differs from the aforementioned prior art methods since embodiments of the invention are applicable not only to axial displacements and strains but also displacements and strains in all directions, displacement ratios, strain ratios and the time-dependent behavior of the aforementioned parameters can be determined and imaged.